Modern analog photonics links require efficient methods of analog modulation with high linearity, commonly defined as high spurious free dynamic range (SFDR). Typically modulation is achieved using either electro-absorption modulators (EAM) in which just as the name implies the absorption coefficient of the device is modulated by the electric field, or electro-optic modulators (EOM) in which the refractive index is modulated and the ensuing phase modulation is converted into optical intensity modulation using an interference scheme, typically a Mach-Zehnder Interferometer (MZI), as shown in FIG. 1. While either EAM or MZI based modulators work very well for digital signals, where linearity is of less concern, the inherent nonlinearity of the modulation characteristics of both modulators reduce the dynamic range of analog photonics links. There have been numerous schemes for linearization of modulators, involving both electronic and optical means and multiple modulators, but their complexity prevents them from being widely used in practical applications. More recently, a relatively simple all-optical linearization scheme for MZI based modulators has been proposed, e.g. see, X. Xie et al, ‘Linearized Mach-Zehnder intensity modulator’, IEEE Photonics Technology Letters, 15(4): pages 531-533, 2003. Linearization was achieved using ring resonators coupled to one or both arms of the MZI. This scheme, the ring-assisted MZI (RAMZI) modulator, shown in FIG. 2, relies on the inherent nonlinearity of the phase transfer characteristics of the ring resonator. When a ring resonator is tuned to anti-resonance its phase modulation characteristics become super-linear (positive 3rd derivative) and the nonlinearity of the MZI modulator, which is sub-linear (negative 3rd derivative), is cancelled, with higher order cancellation requiring more separately driven rings. Cancellation of the third and higher odd order distortion in the transfer characteristics of modulator is the goal of every linearization scheme, including the present one.
The capacity of modern high speed optical communication networks is currently limited by the bandwidth in the telecommunication bands, roughly a few Terahertz. Using the simple on-off keying (OOK) modulation format the capacity of a single fiber thus cannot exceed a few Terabits per second. Currently, long range communication networks are moving to coherent modulation formats that involves altering the phase of the signal. Using the quadrature phase shift keying (QPSK) modulation format with two polarizations increases capacity by a factor of 4. In order to increase the capacity even further one must use more advanced so-called “coherent” modulator formats, such as optical OFDM (orthogonal frequency division multiplexing) and/or multilevel Quadrature Amplitude Modulation (QAM). The higher the level of multilevel modulation, the higher the spectral efficiency (bits/Hz) of the link. However, high levels of multilevel modulation require higher linearity of amplitude modulation; which current modulators do not provide.
There is a need for an increase in the linearity of optical modulators in order to overcome current limitations in performance of analog photonics links and radar technology, to increase the SFDR of such links and systems. In addition, there is a need for linearized modulation techniques in digital multi-level modulation formats, such as OFDM and QAM, in order to increase spectral efficiency of digital optical communication links.